Color CRTs employ an electron gun arrangement that generates three independent electron beams, one for each of the three primary colors of red, green and blue. The electron beams are closely spaced within a common neck portion of the CRT's glass envelope, use a common magnetic deflection yoke for scanning, and are directed through a shadow mask having hundreds of thousands of small electron beam passing apertures. The shadow mask serves as a color selection electrode permitting only a designated electron beam to be incident upon a corresponding color producing phosphor element on the inner surface of the CRT's display screen. In many color CRTs, the three electron beams are arranged horizontally in an inline array.
The CRT's display screen and the shadow mask are arranged in closely spaced relation and are both curved in an axially symmetric fashion. The spherical shape of the display screen and shadow mask gives rise to distortion of the electron-spot triads, or the locations of incidence of the three electron beams on the inner surface of the display screen. The distortion is a foreshortening of the triads in the radial direction resulting in an effective rotation or tilt of the three electron beams particularly in the corners of the display screen. This is shown in FIG. 1 where there is illustrated a plan view of a CRT display screen 32. As shown in the corners of the display screen 32, and with particular reference to the upper right hand corner of the display screen, it can be seen that the triad of the three electron beams 34R (red), 34G (green), and 34B (blue) undergo an angular tilt arising from the geometric distortion of the curved shadow mask and display screen combination. This electron beam tilt reduces color purity tolerance of the video image presented on the display screen.
To correct for this geometric distortion, a conventional vari-bow shadow mask is adapted to accommodate a display panel having a single radius of curvature (1 R or 1.5 R). In the vari-bow shadow mask, the vertical pitch (P.sub.v) or the vertical center-to-center spacing between adjacent shadow mask apertures not in the same vertical column, decreases gradually with increasing X in the horizontal direction. In other words, the vertical spacing between adjacent electron beam passing apertures decreases in proceeding from a vertical centerline of the mask toward one of its lateral edges. A quadratic equation is used for determining P.sub.v values as a function of the horizontal position X on the display screen. Referring to FIG. 2, there is shown a simplified plan view of a conventional color CRT shadow mask 36 showing for the sake of simplicity only a few of the large number of spaced electron beam passing apertures arranged in vertical columns and horizontal rows. Shadow mask 36 is defined by a horizontal X-axis and a vertical Y-axis (shown in the figure as dotted lines), each passing through the center of the shadow mask. Shadow mask apertures 35 in first and second vertically spaced horizontal rows 37 and 38 have a vertical center-to-center spacing of the vertical pitch P.sub.v which is defined by a quadratic equation over the entire surface of the shadow mask. This approach corrects for electron beam tilt for the inner portion of the CRT display screen where the radius of curvature is essential constant. However, with the introduction of high resolution CRTs having flatter display screens, the area outside of the area of fixed radius of curvature suffers from even greater electron beam tilt, particularly in the corners of the generally rectangular display screen. Thus, prior approaches employing a quadratic equation for defining shadow mask aperture vertical pitch have been unable to compensate for electron beam landing tilt over the entire visible area of the display screen, giving rise a reduction in video image color purity tolerance. In many cases, CRT manufacturers have elected to reduce the effective viewing area of the display screen in order to avoid these color purity problems. As a result, the actual viewing area of the CRT is smaller than originally designed. It is, of course, desirable to maximize the viewing area available on a given CRT display screen.
The present invention addresses the aforementioned limitations of the prior art by providing a shadow mask arrangement which compensates for electron beam tilt over both an inner, flatter portion of the CRT's display screen as well as over its outer, more highly curved periphery, and particularly in its corners. The inventive shadow mask arrangement increases video image color purity tolerance in those areas closest to the outer periphery of the display screen without reducing the effective viewing area of the screen.